How stable are diurnal cortisol activity indices in healthy individuals? Evidence from three multi-wave studies

Background: Indices of cortisol activity, including the cortisol awakening response (CAR), diurnal slope, and cortisol output across the day (total daily output), are often studied as mechanistic indicators that could link stress with health. Yet there is a paucity of data speaking to their temporal features, particularly whether they behave in a more state- or trait-like manner across time. Methods: To address this issue, data from 3 studies were used to assess CAR, diurnal slope and total daily output stability over different age groups and time spans: 130 healthy children and adolescents collected salivary cortisol samples 5 times/day (1, 4, 9 and 11. h after wake) over 2 days at 5 visits spaced 6 months apart (Study 1); 147 adolescent girls collected saliva 6 times/day (wake, 1, 4, 9 and 14. h after wake) for 2 days at 3 visits, each a year apart (Study 2); and 47 healthy, primarily middle age adults collected saliva 6 times/day (wake, 1, 4, 9 and 14. h after wake) for 3 days at 4 visits spaced 2-3 months apart (Study 3). Stability was estimated by multilevel model-derived intraclass correlation coefficients (ICCs). Results: Across studies, approximately 50% of the variance in cortisol indices was attributable to day-to-day fluctuations, suggesting state-like properties. Of the indices, total daily output emerged as the most stable over time, followed by diurnal slope and CAR, but stability estimates were generally quite modest regardless of index and sample. Over time spans of >1 year, ICCs were ≤ 13. Conclusions: Most of the variance in CAR, diurnal slope and total daily output reflects day-to-day fluctuation; there was little evidence for more stable trait-like influences. These findings suggest that future research should focus on short-term fluctuations in stress, cortisol and health, as opposed to lengthy disease processes.